Web Server (Computer)
Every website must be hosted on one or more computers (server, web server): the hardware employed to build this computer can be extremely simple or extremely complicated, groups of multiple servers (clusters) might be used, sharing the load of the requests coming from the web. This particular configuration is required when a website gets massive traffic.
On the other hand a web server can be a computer as simple as your home computer, or a computer with a particular case designed for data center usage. These servers can be mounted inside server racks to optimize space. These servers are usually wide, deep and very thin. The width is 19 inches, it’s standard so that servers can be mounted in any server rack.
Rackable servers can have different heights. The base unit the rack Unit, commonly referred to as U, which is 1.75 inches high. Most common sizes for servers are 1U, 2U and 4U depending on the number of the components (disks, CPUs, ram memory sticks, power supplies etc). As far as it regards their functionality there’s no difference between the two types of servers, the difference is only in terms of space required inside the server farm.
Web Server (Software)
If we refer to a web server software we’re talking about that program that actually delivers the content of your website. This software waits for requests of web pages coming from the internet and, when such requests arrive, retrieves the page and pushes it to the browser from which it received the request the request.
This is the functioning in case of static websites, instead if the website is dynamic the web server software uses additional modules and functions to build the content to be delivered.
It is really easier to explain this with an example.